With the recent advances in electronic technology, small portable electronic devices such as camera-integrated VTRs, mobile phone, and laptop computers have been developed, and the development of a secondary battery with small size, light weight, and high energy density is strongly desired as a portable power supply for the devices.
As such a secondary battery to meet this desire, non-aqueous electrolyte secondary batteries which can theoretically generate a high voltage and have high energy density, using a light metal such as lithium, sodium, or aluminum as a negative electrode active material, are expected. Among the non-aqueous electrolyte secondary batteries, a lithium ion secondary battery in which charge and discharge of lithium ions is carried out via the non-aqueous electrolyte is actively researched and developed, because it is expected to achieve a high power and high energy density in comparison with aqueous electrolyte secondary batteries such as the nickel-cadmium battery and the lead-acid battery.
Because the lithium ion secondary battery has a high internal energy, a high degree of safety is required in an abnormal event such as an internal short-circuit or an external short-circuit. For the reason, a polyolefin microporous membrane is used as the separator for safety measures, because it is thought that the polyolefin microporous membrane has a shutdown effect in which the conversion into a nonporous membrane caused by abnormal heating shuts down the current. Even if such safety measures are employed, there is still a possibility that abnormal heating does not stop and causes the shrinkage or melting of the polyolefin microporous membrane, and as a result, a fire of the battery occurs by an internal short-circuit caused by the contact between the electrodes.
In order to increase thermal safety of a lithium ion secondary battery by increasing heat resistance of a separator, it has been proposed that an integration of the polyolefin microporous membrane and a heat-resistant porous material. For example, “a separator for a lithium ion secondary battery characterized by comprising a polyolefin porous membrane and a polyester resin porous membrane” (patent reference 1) and “a separator characterized in that a heat-resistant barrier film is bonded with a liquid holding film and the puncture strength is 400 gf or more” (patent reference 2) were proposed. When these separators are used, a short-circuit caused by shrinkage or melting does not occur, but the battery reaction progresses further, and there is a risk of thermal runaway. In addition, when the voltage is decreased, the battery can be recharged, and there is a risk of runaway of the battery.    [patent reference 1] Japanese Unexamined Patent Publication (Kokai) No. 2002-190291    [patent reference 2] Japanese Unexamined Patent Publication (Kokai) No. 2004-363048